Hearing loss is the most prevalent chronic health condition in the United States with one out of ten Americans suffering some level of loss. Despite the fact that the loss can be mitigated through the use of hearing aids only 25% of affected Americans make use of them. The reasons for this are varied but include cost, physical discomfort, lack of effectiveness in some specific listening situations, societal perception, and unawareness of the hearing loss.
Traditionally hearing loss is diagnosed by a medical specialist using an audiometer. This tool measures the hearing loss parameters of an individual in comparison to a set of reference parameters to develop a hearing loss profile. This profile is then used as the basis for customizing a hearing aid for the individual. These tests are typically performed in a noise-free environment where impediments to hearing are not present.
Calibration of the testing system is required for any results to be useful in mitigating for hearing loss. Mainly, this means that the acoustical pressure delivered to the individual's ear drum must be known for the various settings on the testing machine. Unfortunately this pressure varies from individual to individual even using the same equipment. To correct for this a coupler is typically used where the delivered pressure is known on an absolute basis.
Testing of the individual's hearing loss is performed by producing a pure tone that is delivered to the subject, in a controlled manner as described above, through an earphone in a headset. The individual responds by signaling when he hears the tone. The test operator will increase or decrease the amplification as required for each frequency. The process is repeated until a set number of reversal points have been reached. The individual's hearing threshold is defined as the amplification level where an individual hears the tone 50% of the time. In order to determine the hearing loss profile the hearing threshold of the individual is typically tested at seven different frequencies (125, 250, 500, 1000, 2000, 4000, and 8000 Hz). If the hearing threshold between any two adjacent frequencies (500 to 8000 Hz) exceeds a critical value (typically 20 dB) then a test at an intermediate frequency is performed. This can result in a maximum of eleven frequencies being tested (125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000, and 8000 Hz).
While developing the hearing loss profile in this way permits hearing aid customization that works well in noise free environments, performance may decrease dramatically in the presence of noise. This is a serious limitation as an individual is likely exposed to many environments in which acoustic noise is prevalent, such as a moving automobile or crowded location.
Conventional hearing tests conducted by audiologists establish a general standard of 0–20 decibels over a range of 250 to 8000 Hz as a manual hearing range. Some studies show that within the general hearing range normal-hearing people have a distinct profile and are more sensitive to higher sounds than to lower sounds. See David Pedro Pascoe, Ph. D., “Frequency Responses of Hearing Aids”, doctored dissertation, Central Institute for the Deaf, (1974), pages 12–14, FIG. 2. Those studies suggest that a flat threshold hearing aid profile could be modified to reduce the threshold amplification of low frequencies and/or increase the threshold amplification of high frequencies and thereby generate a fine tuned hearing profile that closely resembles normal-hearing people. The studies indicate that perceived speech is more intelligible when the threshold is fine tuned to the normal hearing profile. In other words, small adjustments are made to alter the threshold from a flat profile in the 0–20 dB range to a frequency dependent profile.
A second problem with the traditional form of testing is its reliance on a specialist to perform the test. This problem is three-fold. First, an individual must make the commitment, in the face of societal pressure, to admit that they have a medical problem. Second, the entire process can be expensive and is not typically paid for by insurance. Third, these specialists are not always readily available to an individual making the process inconvenient and time consuming.
A recent alternate approach to generating an individual's hearing loss profile, independent of an audiometer or specialized operator, is disclosed by Hou in US2002/0183648A1. Also disclosed is the idea of modifying a personal audio system with an individual's hearing loss profile to compensate for that particular loss.
The testing method discussed by Hou involves three basic steps. The first is to test a reference individual, someone who does not suffer from hearing loss, to generate a set of baseline parameters for a specific audio system. In other words, to calibrate the audio system to a reference individual's hearing adjustment profile. Like the traditional hearing test, this method plays pure tones at the same series of frequencies using turning points to determine the hearing threshold. The second step tests the subject individual to determine their hearing parameters in response to the same audio system. The two sets of values are then compared to generate the hearing loss profile. This profile can then be use for various purposes, including for compensation of a personal audio system for the subject individuals hearing loss. Hou's method also allows for manual manipulation of the hearing loss profile to account for possible discrepancies in the testing process.
Hou's approach has a number of limitations. First, because the process is used to generate a hearing loss profile, the audio system used for the testing must still be calibrated, in this case, to the hearing profile of the reference individual. If the system were not calibrated then the results could not accomplish the stated purposes. Second, if the reference and subject individual's physical positions or orientations with respect to the audio source were not identical, comparisons of the testing data could result in an inaccurate hearing loss profile and a correspondingly ineffective compensation of the audio system. Finally, if the background noise varies between the reference and subject individual's tests, the resulting hearing loss profile could be similarly inaccurate, resulting in inappropriate audio system compensation.